fixed typo in gist

inputs/lecture_21.tex

@@ -107,7 +107,7 @@
                  y &\longmapsto & m \beta^{(y)}
             \end{IEEEeqnarray*}
             continuous.
-        \item $\phi \circ S = m \cdot  \phi \circ \sigma$:
+        \item $\phi \circ S = \phi + m \sigma$:
             $Z$ is $T$-invariant.
         \item $[\phi \circ S \circ \gamma] = [\phi \circ \gamma] + [m \sigma \circ \gamma]$
             $\implies [m \sigma \circ \gamma] = 0 \lightning$.